Generally speaking, a pavement consists of the actual ground or subgrade, a subbase overlying the subgrade, a base overlying the subbase and a surface course or wear course on the top. A variety of materials have been used in the construction of pavements including aggregate bound together by a bituminous binder, cementitious mixture including flyash and gypsum, sand, soil cement, blast furnace slag, synthetic aggregates and other similar material depending upon the particular layer.
It has already been proposed to utilize phosphogypsum prepared as a by-product from the manufacture of phosphoric acid by the wet process as fill or base in the construction of pavements and foundations, see French Pat. No. 2,340,405. The French patent discloses a paving composition containing phosphogypsum having a moisture content of between 8 and 23%.
Flyash has also been used in paving compositions, U.S. Pat. No. 3,854,968 to Minnich et al discloses a cementitious mixture composed of flyash, lime, and gypsum used to stabilize soils or to form a subsurface base for highways, runways, etc.
Phosphogypsum is available in large quantities as a by-product of the wet-process production of phosphoric acid, with the formula CaSO.sub.4.2H.sub.2 O. As a rule, 5.1 tons of phosphogypsum are produced for each ton of phosphoric acid produced. For instance, the Florida phosphate industry generates 33 million tons of phosphogypsum annually and only a small fraction (about 700,000 tons) is used for agricultural purposes, see the article by May et al. entitled, "Assessment of Environmental Impacts Associated with Phosphogypsum in Florida," U.S. Dept. of Interior, Bureau of Mines, Report of Investigations 8639, 1982.
Flyash is a finely divided ash residue produced by the combustion of pulverized coal, which ash is carried off with the gases exhausted from the furnace in which the coal is burned and which is collected from those gases usually by means of suitable precipitation apparatus such as electrostatic precipitators. This flyash, which is a siliceous or alumino-siliceous material, having the physical properties of a solid yet fluid-like dust or powder, had less than a 10 percent utility as recently as 1977, a year in which an estimated 45 million tons was produced as a by-product of generating electricity. Flyash could be and was used in negligible amounts in combination with lime to produce a cementitious composition. It was not competitive, however, with the traditional form of cement, popularly known as Portland cement, in part because it required a substantially longer setting up or hardening period.
Therefore, there is urgent need to develop a method of disposing of flyash and phosphogypsum in an environmentally acceptable, economical and productive manner.
Applicant, in accordance with the present invention, has found a novel method for making a load bearing surface having a high load bearing capacity using mixtures of flyash and phosphogypsum.